1. Field of the Invention
The present invention relates generally to substrate cleaning and, more particularly, to methods and apparatus for megasonic cleaning of substrates.
2. Description of the Related Art
Megasonics is a highly advanced, non-contact, cleaning technology used for small-particle-sensitive substrates such as semiconductor wafers in various states of fabrication, flat panel displays, micro-electro-mechanical systems (MEMS), micro-opto-electro-chemical systems (MOEMS), and the like. Megasonic processing typically involves the propagation of acoustic energy through a liquid medium to remove particles from, and clean, a surface of a substrate. The acoustic energy is typically propagated in a frequency range of approximately 0.4 Megahertz (MHz) to 1.5 MHz, inclusive, with “megasonic” typically defined as greater than 0.7 MHz. The liquid medium can be deionized water or any of a plurality of substrate cleaning chemicals. The propagation of acoustic energy through a liquid medium achieves non-contact substrate cleaning chiefly through pulsating bubbles and/or cavitation, microstreaming, electrical field effects, and chemical reactions enhancement when chemicals are used as the liquid medium.
FIG. 1A is a diagram of a batch substrate megasonic cleaning system 10. Tank 11 is filled with a cleaning solution 16 of deionized water or other substrate cleaning chemicals as desired. A substrate carrier 12, typically a cassette of substrates, includes a batch of substrates 14 to be cleaned. Transducer 18 generates the acoustic energy that is propagated through the cleaning solution 16. The megasonic energy, with or without appropriate chemistry to control particle re-adhesion, achieves substrate cleaning through pulsating bubbles and/or cavitation, microstreaming, electrical field effects, and chemical reactions enhancement if chemicals are used. Batch substrate megasonic cleaning tends to require lengthy processing times, and also can consume excessive volumes of chemicals. Additionally, consistency and wafer-to-wafer control are difficult to achieve. Such conditions as “shadowing” and “hot spots” are common in batch, and other, substrate megasonic processing. Shadowing occurs due to reflection and/or constructive interference of megasonic energy, and is compounded with the additional substrate surface area of multiple substrates. The occurrence of hot spots, primarily the result of constructive interference in addition to reflection, can also increase with additional multiple-substrate surface areas. One solution to these drawbacks of batch substrate megasonic processing has been the use of increased or higher megasonic energy as well as multiple transducer arrays. Also, a single substrate megasonic cleaning system evolved in response to these and other concerns.
FIG. 1B is a diagram of a single substrate megasonic cleaning system 20. The single substrate megasonic cleaning system 20 includes a tank 22 filled with a cleaning solution 28 of deionized water or substrate cleaning chemicals according to desired processing. A single substrate 24 is submerged in the cleaning solution 28 of tank 22. Transducer 26 generates the acoustic energy that is propagated through the cleaning solution 28. As in the batch substrate processing system, the megasonic energy, with or without appropriate chemistry to control particle re-adhesion, achieves substrate cleaning through pulsating bubbles and/or cavitation, microstreaming, electrical field effects, and chemical reactions enhancement if chemicals are used. A drawback of the single substrate megasonic cleaning system 20 results from the significantly smaller size of the tank 22. Effective substrate processing removes particles that remain inside the tank 22 requiring that the cleaning fluid 28 be replaced or recirculated and filtered regularly.
FIG. 1C is a diagram of single substrate, nozzle-type megasonic cleaning system 30. Nozzle 31, coupled with transducer 38, supplies a fluid stream 34 to the surface of a substrate 32 forming a meniscus 36. Transducer 38, generates the acoustic energy 40 that travels with the fluid stream 34 and is propagated through the meniscus 36 to the surface of substrate 32 as the fluid stream 34 flows through the nozzle 31 maintaining the meniscus 36. Megasonic energy 40 propagating through meniscus 36 effects substrate 32 cleaning. Additionally, substrate 32 is rotated to achieve uniform cleaning of an entire surface of substrate 32. In FIG. 1C, directional arrow 42 indicates substrate 32 rotation.
FIG. 1D shows another single substrate cleaning system 50 in which a meniscus 36 is formed and maintained on the surface of a substrate 32 by a fluid stream 34 delivered by any of a plurality of supply methods. A local area transducer 38 generates acoustic energy 40 that is propagated through the meniscus 36 to the surface of the substrate. Substrate 32 is rotated 42 to achieve uniform cleaning of an entire surface of substrate 32.
Either single substrate megasonic cleaning system illustrated in FIGS. 1C and 1D is limited to a single surface megasonic cleaning process at a time, and in particular, the single substrate cleaning system 50 illustrated in FIG. 1D requires a high fluid flow and fluid volume to maintain the meniscus 36 and the necessary fluid medium contact between transducer 38 and substrate 32. Additionally, a fairly high megasonic energy is needed to clean the substrate 32, which in turn, may cause damage to the surface of the substrate 32. The high energy required also necessitates cooling of the transducer 38, typically increasing the fluid flow and volume requirements. Due to cost and effluent handling requirements, substrate processing using a cleaning chemistry other than deionized water is impractical.
As is known, the cleaning chemistries for single substrate cleaning processes are typically highly reactive and often require application at elevated temperatures to provide effective cleaning at low cleaning duration, particularly for post etch cleaning applications. Each of the single substrate cleaning configurations described above use batch heating systems with chemical re-circulation, or in the case of a single substrate, nozzle-type megasonic cleaning system 30, batch heating with heated delivery lines so that the fluid temperature of the meniscus 36 is maintained to optimally clean the substrate 32 surface.
Additionally, as particle tolerance specifications continue to become more and more stringent, it becomes increasingly desirable, if not necessary, to perform megasonic processing on both the active and back-side surfaces of a substrate.
In view of the foregoing, there is a need for a method and apparatus to provide a single substrate megasonic processing configuration that is capable of providing consistent, controllable, and effective megasonic processing. Methods and apparatus for single substrate megasonic processing should be configured to meet or exceed particulate requirements, and should be easily configurable to existing substrate processing systems.